Don't call me Mr Darcy

Dominic Cooper has played a succession of cads in wigs and britches, the latest opposite Keira Knightley in The Duchess. But the rising star won't be pigeonholed. He tells Stuart Jeffries why he's just as happy wearing a pink spandex leotard

The following correction was printed in the Guardian's Corrections and clarifications column, Wednesday September 3 2008

In the article below we reminded Dominic Cooper that Chatsworth House served as Pemberley in the scene in Pride and Prejudice where Colin Firth emerges from a lake. We have been reminded that the scene was filmed at Lyme Park in Cheshire.

I don't," says Dominic Cooper, dipping a biscuit coquettishly into his cappuccino froth, "take roles where I keep my clothes on." He is joking. Or is he? Even now I can't help but think of Keira Knightley lowering her embonpoint on to his buff chest in his new costume-drama romp, The Duchess. Not to mention that scene in Mamma Mia! in which he bobs towards his fiancee across the Aegean, astride a jet-ski, half naked but without a hint of torso jiggle. Not since Daniel Craig emerged from the sea in those swimming trunks, has British masculinity been so out and proud on celluloid as it has been in Cooper's recent work.

But Cooper does more than nude torso. "In any photo shoot," he says, archly raising his left eyebrow, "I have to be covered in lube." He's responding gamely to my what-were-you-thinking-of question about his recent shoot for the gay lifestyle magazine Attitude in which he appeared in an array of moist poses. "Actually, it wasn't lube, it was water," he says. "The photographer had an idea, which I thought was good aesthetically, that there would be a mist of water between him and me. But when I got to the shoot there was this bloke on a ladder with a bucket of water which he kept throwing over me. I only got mildly aggressive towards the end."

Cooper, 30, is lucky enough to unite two distinct demographics in drooling, a feat that would make his contemporaries at the London Academy of Music and Dramatic Art (from which he graduated in 2000) envious. While Attitude describes him as "tall and lithe and tanned with big brown eyes and a sexual charisma that envelops you like a kidnapper's sack over your head", the Daily Mail reckons Cooper is the "new Mr Darcy". In the latter fantasy, Cooper is a similarly smouldering but younger Colin Firth, the man who made Sunday night TV viewers come over all unnecessary during the BBC's adaptation of Pride and Prejudice more than a decade ago.

Does he mind being a sex symbol? "For the three years I was in school training to be an actor, I was told, 'It's very unlikely you'll work at all on the stage or in film', so I feel I have to take all the opportunities I can. I'm terribly lucky. There's a constant fear that it could end at any moment. To stay lucky I have to work at it." In a wet T-shirt? "If that's what it takes," he says, then checks himself. "You know, the problem with this whole career is that you get sucked into a whirlwind and other people make choices for you."

His 12-year relationship with Joanna Carolan, PA to playwright Harold Pinter, has been one casualty of Cooper's whirlwind success. "We're no longer together and that is probably because I've spent too many hours concentrating on my career. It's very tough on a relationship." The tabloids told a more lurid tale. "Hunk who plays Sense And Sensibility's Willoughby is a cad in real life too" ran the Daily Mail's headline above a story reporting the rumours that Cooper had been seen kissing his on-screen lover in Mamma Mia!, 22-year-old Amanda Seyfried, at the Cafe Rouge in Bath. How could he, the Mail seethed. Good point: Cafe Rouge is a rubbish place for a date.

Carolan, he told one interviewer earlier this year, is a brilliant actor but made a decision to change career two years ago. "I think she just got fed up," he told his interviewer. "There's only so much you can do of trying, finding yourself very close to getting a part and then not getting it."

Did his success and her relative failure put strains on their relationship? "It wasn't that," says Cooper. Instead, he offers a mea culpa. "There are probably only a certain number of people who can understand or tolerate how long a job will take and what demands it puts on you. And why should they? It breeds a strange kind of selfishness immersing yourself in a character for so long."

In his new film, The Duchess, Cooper plays Charles Grey, the future Whig prime minister, who comes to Chatsworth House in Derbyshire to get it on adulterously with Knightley's Georgiana, Duchess of Devonshire. It was Chatsworth House, I remind Cooper, that served as Pemberley in that scene in Pride and Prejudice where Firth emerges from the lake, wet shirt clinging to his chest, and turns the knees of Jennifer Ehle's Elizabeth Bennet to jelly.

"I never did get to swim in the lake," Cooper says with the regret of a man missing an opportunity to appear damp on camera. But he reminds me that he has learned a lot first hand from Firth about how to make an honourable career from being perceived as costume-drama crumpet.

"I've literally been around the world promoting Mamma Mia! and most of that time I was sitting next to Colin on flights to some press junket. So I know all about the Mr Darcy stereotype and how he avoided being pigeonholed while using that part to help his career. "We want to pigeonhole things and people, but it is absurd to regard me just as a furry wig-and-britches actor."

But he did play Jane Austen's Willoughby in the BBC adaptation of Sense and Sensibility earlier this year, and now he plays Grey in The Duchess and soon we're going to see him as Steerforth in a TV adaptation of Dickens' David Copperfield, which may well feature britches. "Yes, but I was also wearing a pink spandex leotard in Mamma Mia!"

To be fair, Cooper is not just crumpet. He is regarded as one of Britain's great young acting hopes, often ranked with James McAvoy, Ben Whishaw, Rebecca Hall, Emily Blunt and Hayley Atwell. After graduation, the Greenwich-born Cooper was fortunate enough to get a role in Mark Ravenhill's play Mother Clap's Molly House at the National Theatre. He worked at the Royal Shakespeare Company, the Royal Court and had some minor TV and film roles before he returned to the National to make his mark with His Dark Materials and The History Boys. In Alan Bennett's play, he was Dakin, the randy sixth former with impish eyebrows whose bedroom eyes promised his teachers afternoons of career-ruining bliss. It was some calling card.

"I never thought it would be a huge success, but I really wanted the part because it was Alan Bennett." But it was a success: the production transferred from London to Broadway and was then made into a triumphant film. He keeps in touch with Bennett. "I went round to his house for tea the other day. He's such a lovely man."

How does he feel about what George Steiner no doubt calls "the erotics of pedagogy"? "The who?" he replies, nearly gagging on his coffee. Teachers copping a feel, as Richard Griffiths did in The History Boys. "Oh right. I'd better be careful here because I don't want to make it seem I'm endorsing copping a feel by teachers, but it was certainly a story that needed to be told."

Quite so. Whether that is true of his latest film is less certain. The Duchess uses Amanda Foreman's bestselling biography of Georgiana, the Duchess of Devonshire, to spin a fictionalised history of her life. Cunningly, perhaps cynically, it stresses the links between her 18th-century menage and the Charles, Di and Camilla love triangle (Georgiana was an ancestor of Lady Diana Spencer). The movie tie-in edition of the book even has the blurb "there were three people in her marriage", alluding to the line Diana used when interviewed by Martin Bashir. In Georgiana's case the three people were the Duchess (Knightley), the Duke (Ralph Fiennes), and his mistress, who supplied him with a clutch of illegitimate sons. Grey was the fourth banana in this marriage - the lover who was both, if one wants to press the Diana parallel, Dodi Fayed and Captain James Hewitt united in one neat package.

What is it like to rip a bodice - giggle or chore? "It's harder than you think," says Cooper, who, it must be said, is endearingly game to answer any daft question I fire at him. "Keira was lovely to act with, but there were something like 73 layers of petticoat to get through. You wonder that they managed to get any action at all in those days."

We will next see Cooper alongside Emma Thompson in the coming-of-age drama An Education, a film based on an original script by Nick Hornby. He took the role of a smooth-talking early 60s London playboy after Orlando Bloom declined it. "'He replaces Orlando Bloom.' I love that," Cooper says. "You know you've arrived when headlines say that. The story is wonderful: about this middle-class girl who is seduced by my character, who's a kind of glamorous Rachman-type criminal who charms old ladies over tea into giving him their houses. I'm slimy, repellent and glamorous."

Cooper yearns to get back to the stage and hopes to appear in the National's new production of Racine's Phèdre next year. In the meantime, he's enjoying being home. "It's really exciting going round the world with Meryl Streep, Pierce Brosnan and Colin Firth, being treated like royalty, but I really relish being back in London, playing football and getting kicked shitless by mates in Dulwich I've known since I was five. It's a relief to be treated as a numbskull, you know?."

On this week's podcast, Michael Winterbottom on why the Raindance open film competition Short Circuit is well worth entering. Plus Xan Brooks is back to review this week's releases with Jason, and we meet the director of The Duchess, Saul Dibb